1. Field of the Invention
The present invention relates to an image forming apparatus such as a color printer, a color copying machine or a color facsimile apparatus employing an electrophotographic process.
2. Related Background Art
The conventional color image forming apparatus (for forming an image with two or more colors) employing an electrophotographic recording method generally employs non-magnetic toner as the developer for all the colors (yellow, magenta, cyan and black). Such non-magnetic toners are further classified into two-component non-magnetic toner and one-component non-magnetic toner. The former, being collectible by magnetic force, allows one to prevent contamination of the interior of the apparatus by the scattered toner. On the other hand, the apparatus tends to become heavy because of the use of magnetic carrier or a developing magnet, so that such toner is unsuitable for a compact desk-top printer or the like employing a cartridge structure in the developing unit and associated mechanisms for easy maintenance. This drawback can be resolved by the apparatus employing the latter one-component non-magnetic toner, as exemplified in FIG. 5.
FIG. 5 is a longitudinal cross-sectional view of a color printer of multiple transfer type, employing the one-component non-magnetic toner. At the approximate center of a main body M, there are provided a photosensitive drum (photosensitive member) 1 serving as an image bearing member, a charging roller 2 and a cleaning unit 3. To the right of said photosensitive drum 1 there is provided a developing unit (developing means) 4, which is provided with four non-magnetic developing units 4a, 4b, 4c and 4d respectively containing one-component non-magnetic toners of yellow (Y), magenta (M), cyan (C) and black (Bk) colors, and a support member (rotary member) 9 supporting and rotating said developing units (hereinafter often referred to as rotary developing unit).
The developing units 4a-4d are adapted to rotate, about a rotary axis 9a of the support member 9, in a direction R9 along circular rails M1 provided on both ends of the main body M, and are so constructed as to maintain the horizontal position during rotation, whereby developing apertures 5a, 5b, 5c and 5d remain horizontal. Said developing units 4a-4d are provided, as shown in FIG. 6, with coating rollers 6a, 6b, 6c and 6d, and toner limiting members 7a, 7b, 7c and 7d, and, with the rotation of developing rollers 8a, 8b, 8c and 8d in a direction indicated by arrows, toner is coated by the coating rollers 6a-6d onto the developing rollers 8a-8d and necessary triboelectric charges are given by the toner limiting members 7a-7d. Said toner limiting members 7a-7d can be composed for example of nylon (in the case of charging the toner negatively), or of silicone rubber (in the case of charging the toner positively). More specifically said toner limiting members are preferably composed of a material chargeable to a polarity opposite to the desired polarity of the toner. The peripheral speed of the developing rollers 8a-8d is preferably within a range from 1.0 to 2.0 times of that of the photosensitive drum 1. The developing units 4a-4d supported by the support member 9 are so moved that the developing apertures 5a-5d are constantly opposed to the photosensitive drum 1. Such moving method is detailedly disclosed for example in Japanese Laid-Open Patent Applications 50-93437, and U.S. Pat. Nos. 4,743,938, 4,697,915, 4,620,783, and 4,622,916.
To the left of the photosensitive drum 1 in FIG. 5, there is provided a transfer drum 10 for supporting a recording material (not shown) and serving as transfer means for transferring the toner image, formed on the photosensitive drum 1, onto said recording material. In the upper part of the main body M, there is provided an optical unit (latent image forming means) 16 composed for example of a laser diode 11, a polygonal mirror 13 rotated by a high-speed motor 12, a lens 14 and a mirror 15.
The above-mentioned photosensitive drum 1 is composed of an aluminum cylinder of a diameter of 40 mm, with a photoconductor consisting of an organic photoconductor (OPC) coated on the external periphery, but said OPC may be replaced for example by a-Si, CdS or Se.
At the image forming operation, the photosensitive drum 1 is rotated in a direction R1 by drive means (not shown) with a peripheral speed of 100 mm/sec., whereby the drum surface effects endless movement. The thus-rotated photosensitive drum 1 is charged by the charging roller 2, receiving a DC voltage of -700 V overlapped with a peak-to-peak AC voltage of 1500 V with a frequency of 700 Hz, whereby the surface of the photosensitive drum 1 is uniformly charged to ca. -700 V.
The laser diode 11 of the optical unit 16 emits a laser beam in response to the entry of signals according to the yellow image pattern, and said laser beam irradiates the photosensitive drum 1 through an optical path L formed by the polygon mirror 13, lens 14, mirror 15 etc., whereby the potential of the photosensitive drum 1 drops to ca. -100 V in the irradiated portion and a latent image is thus formed.
The photosensitive drum 1 further rotates in the direction R1, whereby the latent image on the surface is subjected to the deposition of yellow toner by the yellow developing unit 4a of the developing device 4 and is rendered visible as a toner image. Prior to the developing operation, the developing unit 4a is brought to a developing position opposed to the image bearing member, by the rotation of the support member 9 in a direction R9. Subsequently the toner image on the photosensitive drum 1 is transferred onto the recording material supported on the transfer drum 10.
Said recording material is supplied from a cassette 17 by means of a pick-up roller 18 in synchronization with the toner image on the photosensitive drum 1, and is adhered to the transfer drum 10. Said transfer drum 10 is composed of a metal cylinder 19 of a diameter of 156 mm, an elastic layer 20 of a thickness of 2 mm and an upper PVDF layer 21 of a thickness of 100 .mu.m formed in succession thereon, and is rotated in a direction R10, with a peripheral speed substantially the same as that of the photosensitive drum 1. When the recording material is supplied, as explained above, onto said transfer drum 10, the front end of the recording material is supported by a gripper 22. Between the transfer drum 10 and the photosensitive drum 1 a transfer voltage is applied by a power source (not source), whereby the yellow toner image on the photosensitive drum 1 is transferred onto the recording material, and the entire recording material is adhered along the surface of the transfer drum 10 by the charge injection into the recording material. If necessary, the recording material may be adhered in advance to the surface of the transfer drum 10 by means of a roller 23.
The above-explained image forming process from the charging to the image transfer is repeated for the remaining three colors magenta, cyan and black, to obtain a four-color image on the recording material. The recording material after the toner image transfer is separated from the surface of the transfer drum 10 by a separating charger 26 and a separating finger 24, then is subjected to fixation of the toner images by fusion in a known fixing unit 25 utilizing heat and pressure, and is discharged, as the final product, from the main body M.
On the other hand, the photosensitive drum 1 is subjected to cleaning of the remaining toner on the surface by a cleaning unit 3 utilizing a fur brush or a blade in the already known manner, and is used again in a next image forming process starting from the charging step. Also the unnecessary toner remaining on the transfer drum 10 is removed by a transfer cleaning unit 7 equipped for example with a fur brush or a web, and, if charge remains on the transfer drum 10 after the separation of the recording material, such charge is eliminated by a charge eliminating roller 28.
In the image forming apparatus explained above, all the toners of yellow, magenta, cyan and black colors employed for the development of latent images are all non-magnetic toners. In contrast to the magnetic toner which is controlled in movement by a magnetic field generated for example by a magnetic developing member incorporating a developing magnet, such non-magnetic toner tends to scatter easily, thus causing contamination of the interior of the main body M. For preventing such contamination, there have been employed various means such as determination of the position of a cooling fan for the electric systems in consideration of the air flow within the main body M, or an exclusive fan for collecting the scattered toner, or a complex scattering preventive mechanism such as shutters for closing the developing apertures 5a-5d at the switching of the developing units 4a-4d.
However, if such toner scattering cannot be prevented completely, the contamination of the interior of the main body M is unavoidable in prolonged use of the image forming apparatus, and, if in particular the optical unit 16 constituting the latent image forming means is smeared with the toner, there may result partial interruption of the optical path L or a deficiency in the amount of the laser beam in the exposure (formation of latent image), leading directly to an image defect such as a deficient density in the image development. Such toner scattering hinders the increase in image forming speed of the apparatus since it becomes severer with such speed increase.